1. Field of the Invention
The present invention pertains to apparatus for controlling fluid flow relative to fluid pressure systems. More particularly, the present invention is related to apparatus for controlling fluid communication with a fluid pressure system in response to changing conditions relative to such system. The present invention finds particular application to a type of apparatus or system typically referred to as a "tensioner," and is well adapted to those tensioners which are used offshore in connection with marine riser pipe structures.
2. Description of the Prior Art
In connection with the drilling and production of offshore oil and gas wells, various operations are typically conducted from a support structure, such as a drill ship or semisubmersible platform, which rises and falls with the wave action. One or more wellheads are located on the floor of the body of water generally beneath such a structure. For each such wellhead, a package of equipment, for example a stack of blowout preventers, is run into place on a string of relatively large diameter pipe known as a riser pipe. After this running in operation, the riser pipe is left in place so that it extends upwardly from the wellhead area to a point above the surface of the water and near the support platform whereby it may serve as a rough guide for other strings of apparatus which must, from time to time, be lowered to the wellhead and/or into the well.
The riser pipe, which is anchored on its lower end at the wellhead structure, must be supported with respect to the platform or other support structure for several reasons, including the prevention of collapse of the riser pipe under its own weight as well as the prevention of excessive swaying motion of the riser pipe in the water. Accordingly, it is customary to support the riser pipe with respect to the platform by placing it under considerable tension, the magnitude of the tension load typically exceeding the weight of the riser pipe.
Maintaining the aforementioned tension load at a given value, or at least within a given range of values, is very difficult because of the heaving of the platform or other support structure due to wave action, for example. Thus, the tension cannot be applied by a static or fixed system. Tensioners are known for maintaining a tension load between riser pipe and a support structure in the presence of relative motion between the two.
A typical tensioner fluid pressure system is illustrated schematically in FIG. 2, and includes a tensioner shown in part generally at 10 comprising a piston 12 and cylinder 14 interconnected between the offshore platform and the riser pipe by means of a cable or other flexible line 16 seated in a sheave system 18 movable with the piston or cylinder. Thus, relative movements of the platform and riser pipe tend to cause corresponding relative reciprocation of the piston 12 and cylinder 14, and conversely, reciprocation of the piston and cylinder tend to cause, or at least permit, relative movement of the two interconnected offshore structures.
High pressure fluid communicated from a source 20 is applied against the piston 12 in one end of the cylinder 14, and it is the force of this pressurized fluid which ultimately supports the riser pipe with respect to the platform and applies the desired tension. The high pressure fluid, or at least a portion thereof, is compressible. More specifically, the body of high pressure fluid may be comprised entirely of a gas, or it may be a suitable liquid, such as oil, backed by a volume of pressurized gas communicated by means of a flow line 22 from a source of pressurized gas (not shown). Such application of high pressure fluid permits reciprocation of the piston 12 and cylinder 14, so as to accommodate relative movement of the platform and riser pipe, while still maintaining the tension load on the riser pipe within a given range of values. The application of such pressurized gas to the gas-liquid accumulator 20 also minimizes the effect of the position of the cylinder 14 on fluid pressure applied thereto, and thus to the tension load maintained by the piston and cylinder arrangement 10.
In order to prevent the piston 12 from slamming or jolting action when it reciprocates away from the high pressure end of the cylinder 14, a lower pressure balancing fluid from a source 24 is admitted into the opposite end of the cylinder. Such low pressure fluid may be liquid backed by pressurized gas communicated to the source 24 by a flow line 25 from a source of pressurized gas (not shown). The pressurized fluid may flow into and out of the cylinder 14 to permit the necessary reciprocations of the piston 12, but the low pressure fluid flow rate is controlled by a throttle 26 or the like which slows the piston speed, at least near the end of its stroke, so as to avoid undesirable slamming or jolting during normal operational reciprocations. The maintenance of low pressure fluid within the cylinder 14 also maintains the cylinder full for corrosion protection and lubrication. A manual shut-off valve 28 facilitates maintenance of the system.
A circumstance of concern in connection with the type of tensioner described above occurs when the riser pipe, or a portion thereof, is suddenly disconnected from the wellhead apparatus while the tensioners are still engaged. Such disconnections may occur due to accidents, for example the failure of the riser pipe itself or of some related wellhead apparatus, or through the operation of emergency disconnect systems which are used to disconnect the riser pipe, in the event of severe weather, for example. Such occurrences cannot be completely avoided, and it is not practicable to disengage the tensioners before disconnection of the riser pipe in every such instance. In tensioning the riser pipe, the system applies a large vertical upward force. If the riser pipe is disconnected, the tensioners will continue to apply this force, but the riser pipe, no longer anchored to the wellhead, will be raised upwardly toward the platform. The riser pipe will, in effect, be suddenly jerked upwardly at a relatively high rate of speed. The throttling apparatus 26 may suffice to cushion the advancing piston 12 toward the end of its stroke during normal operational reciprocation, but will not suffice to control the extremely high speeds and forces which prevail when the heavy and highly tensioned riser pipe is suddenly disconnected from the wellhead. The heavy riser pipe can gather sufficient momentum that the pipe will ultimately collide with the platform and may cause serious damage or injury. The particulars of such collision will vary depending upon the location of the support platform with respect to the wave crests and troughs at the time of disconnection of the riser pipe. However, it can be shown that disconnection, at whatever point, can result in a dangerous and expensive collision. As drilling continues at greater and greater depths, riser pipe structures become heavier, and the potential magnitude of such collisions increases.
It is desirable to provide a system for controlling the recoil of a riser in the event of an emergency disconnect of the riser from the underground structure, wherein the riser may be permitted to decelerate to a safe velocity generally toward the surface of the body of water by relieving the tension load on the riser. Such a system should also permit full stroke, normal reciprocation of the tensioner system. Further, such a system should preferably operate automatically, in response to the condition of the riser in the event of such a disconnect.